Substituted triazolo-pyridazine derivative, pharmaceutical compositions made therefrom

ABSTRACT

7-(1,1 -Dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine, and pharmaceutically acceptable salts thereof are selective ligands for GABA A  receptors useful in the treatment of disorders of the central nervous system.

This is an application under 35 U.S.C. 371 of PCT/GB99/01897 and claims the benefit of Great Britain Application No. 9813576.7 filed Jun. 24, 1999.

The present invention relates to a substituted triazolo-pyridazine derivative and to its use in therapy. More particularly, this invention is concerned with a particular substituted 1,2,4-triazolo[4,3-b]pyridazine derivative which is a GABA_(A) receptor ligand and is therefore useful in the therapy of deleterious mental states.

Receptors for the major inhibitory neurotransinitter, gamma-aminobutyric acid (GABA), are divided into two main classes: (1) GABA_(A) receptors, which are members of the ligand-gated ion channel superfamily; and (2) GABA_(B) receptors, which may be members of the G-protein linked receptor superfamily. Since the first cDNAs encoding individual GABA_(A) receptor subunits were cloned the number of known members of the mammalian family has grown to include at least six α subunits, four β subunits, three γ subunits, one δ subunit, one ε subunit and two ρ subunits.

Although knowledge of the diversity of the GABA_(A) receptor gene family represents a huge step forward in our understanding of this ligand-gated ion channel, insight into the extent of subtype diversity is still at an early stage. It has been indicated that an α subunit, a β subunit and a γ subunit constitute the minimum requirement for forming a fully functional GABA_(A) receptor expressed by transiently transfecting cDNAs into cells. As indicated above, δ, ε and ρ subunits also exist, but are present only to a minor extent in GABA_(A) receptor populations.

Studies of receptor size and visualisation by electron microscopy conclude that, like other members of the ligand-gated ion channel family, the native GABA_(A) receptor exists in pentameric form. The selection of at least one α, one β and one γ subunit from a repertoire of seventeen allows for the possible existence of more than 10,000 pentameric subunit combinations. Moreover, this calculation overlooks the additional permutations that would be possible if the arrangement of subunits around the ion channel had no constraints (i.e. there could be 120 possible variants for a receptor composed of five different subunits).

Receptor subtype assemblies which do exist include, amongst many others, α1β2γ2, α2β2/3γ2, α3βγ2/3, α2βγ1, α5β3γ2/3, α6βγ2, α6βδ and α4βδ. Subtype assemblies containing an α1 subunit are present in most areas of the brain and are thought to account for over 40% of GABA_(A) receptors in the rat. Subtype assemblies containing α2 and α3 subunits respectively are thought to account for about 25% and 17% of GABA_(A) receptors in the rat. Subtype assemblies containing an α5 subunit are expressed predominantly in the hippocampus and cortex and are thought to represent about 4% of GABA_(A) receptors in the rat.

A characteristic property of all known GABA_(A) receptors is the presence of a number of modulatory sites, one of which is the benzodiazepine (BZ) binding site. The BZ binding site is the most explored of the GABA_(A) receptor modulatory sites, and is the site through which annxiolytic drugs such as diazepam and temazepam exert their effect. Before the cloning of the GABA_(A) receptor gene family, the benzodiazepine binding site was historically subdivided into two subtypes, BZ1 and BZ2, on the basis of radioligand binding studies. The BZ1 subtype has been shown to be pharmacologically equivalent to a GABA_(A) receptor comprising the α1 subunit in combination with β subunit and γ2. This is the most abundant GABA_(A) receptor subtype, and is believed to represent almost half of all GABA_(A) receptors in the brain.

Two other major populations are the α2βγ2 and α3βγ2/3 subtypes. Together these constitute approximately a further 35% of the total GABA_(A) receptor repertoire. Pharmacologically this combination appears to be equivalent to the BZ2 subtype as defined previously by radioligand binding, although the BZ2 subtype may also include certain α5-containing subtype assemblies. The physiological role of these subtypes has hitherto been unclear because no sufficiently selective agonists or antagonists were known.

It is now believed that agents acting as BZ agonists at α1βγ2, α2βγ2 or α3βγ2 subunits will possess desirable anxiolytic properties. Compounds which are modulators of the benzodiazepine binding site of the GABA_(A) receptor by acting as BZ agonists are referred to hereinafter as “GABA_(A) receptor agonists”. The α1-selective GABA_(A) receptor agonists alpidem and zolpidem are clinically prescribed as hypnotic agents, suggesting that at least some of the sedation associated with known anxiolytic drugs which act at the BZ1 binding site is mediated through GABA_(A) receptors containing the α1 subunit. Accordingly, it is considered that GABA_(A) receptor agonists which interact more favourably with the α2 and/or α3 subunit than with α1 will be effective in the treatment of anxiety with a reduced propensity to cause sedation. Also, agents which are antagonists or inverse agonists at α1 might be employed to reverse sedation or hypnosis caused by α1 agonists.

SUMMARY OF THE INVENTION

The present invention is directed to a compound according to Formula (I) and pharmaceutically acceptable salts thereof that are GABA-A Alpha 2 and/or 3 ligands useful in the treatment of disorders of the central nervous system:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of the adsorption/desorption isotherm of Compound I.

FIG. 2 is a graphical comparison of the XRPD diffractograms of the anhydrous Compound I polymorph A and of the dihydrate of polymorph A.

FIG. 3 is a graphical comparison of the XRPD diffractograms of anhydrous compound I polymorphs A, B, C, and D; and of the methanol and ethanol solvates.

DETAILED DESCRIPTION OF THE INVENTION

The compounds of the present invention, being selective ligands for GABA_(A) receptors, are therefore of use in the treatment and/or prevention of a variety of disorders of the central nervous system. Such disorders include anxiety disorders, such as panic disorder with or without agoraphobia, agoraphobia without history of panic disorder, animal and other phobias including social phobias, obsessive-compulsive disorder, stress disorders including post-traumatic and acute stress disorder, and generalized or substance-induced anxiety disorder; neuroses; convulsions; migraine; depressive or bipolar disorders, for example single-episode or recurrent major depressive disorder, dysthymic disorder, bipolar I and bipolar II manic disorders, and cyclothymic disorder; psychotic disorders including schizophrenia; neurodegeneration arising from cerebral ischemia; attention deficit hyperactivity disorder; and disorders of circadian rhythm, e.g. in subjects suffering from the effects of jet lag or shift work.

Further disorders for which selective ligands for GABA_(A) receptors may be of benefit include pain and nociception; emesis, including acute. delayed and anticipatory emesis, in particular emesis induced by chemotherapy or radiation, as well as post-operative nausea and vomiting; eating disorders including anorexia nervosa and bulimia nervosa; premenstrual syndrome; muscle spasm or spasticity, e.g. in paraplegic patients; and hearing loss. Selective ligands for GABA_(A) receptors may also be effective as pre-medication prior to anaesthesia or minor procedures such as endoscopy, including gastric endoscopy.

WO 98/04559 describes a class of substituted and 7,8-ring fused 1,2,4-triazolo[4,3-b]pyridazine derivatives which are stated to be selective ligands for GABA_(A) receptors beneficial in the treatment and/or prevention of neurological disorders including anxiety and convulsions.

The present invention provides a particular triazolo-pyridazine derivative, and pharmaceutically acceptable salts thereof, which possess desirable binding properties at various GABA_(A) receptor subtypes. The compounds in accordance with the present invention have good affinity as ligands for the α2 and/or α3 subunit of the human GABA_(A) receptor. The compounds of this invention interact more favourably with the α2 and/or α3 subunit than with the α1 subunit. Indeed, the compounds of the invention exhibit functional selectivity in terms of a selective efficacy for the α2 and/or α3 subunit relative to the α1 subunit.

The compounds of the present invention are GABA_(A) receptor subtype ligands having a binding affinity (K_(i)) for the α2 and/or α3 subunit. as measured in the assay described hereinbelow, of less than 1 nM. Furthermore, the compounds in accordance with this invention exhibit functional selectivity in terms of a selective efficacy for the α2 and/or α3 subunit relative to the α1 subunit. Moreover, the compounds according to the present invention possess interesting pharmacokinetic properties, notably in terms of improved oral bioavailability.

The present invention provides 7-(1,1-dimethylethyl)-6(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine of formula I:

or a pharmaceutically acceptable salt thereof.

The compounds in accordance with the present invention are encompassed within the generic scope of WO 98/104559. There is, however, no specific disclosure therein of the compound of formula I as depicted above, or pharmaceutically acceptable salts thereof.

For use in medicine, the salts of the compound of formula I above will be pharmaceutically acceptable salts. Other salts may, however, be useful in the preparation of the compound of formula I or of its pharmaceutically acceptable salts. Suitable pharmaceutically acceptable salts of the compound of formula I include acid addition salts which may, for example, be formed by mixing a solution of the compound of formula I with a solution of a pharmaceutically acceptable acid such as hydrochloric acid, sulphuric acid, methanesulphonic acid, fumaric acid, maleic acid, succinic acid, acetic acid, benzoic acid, oxalic acid, citric acid, tartaric acid, carbonic acid or phosphoric acid.

Also provided by the present invention is a method for the treatment and/or prevention of anxiety which comprises administering to a patient in need of such treatment an effective amount of the compound of formula I as depicted above or a pharmaceutically acceptable salt thereof.

Further provided by the present invention is a method for the treatment and/or prevention of convulsions (e.g. in a patient suffering from epilepsy or a related disorder) which comprises administering to a patient in need of such treatment an effective amount of the compound of formula I as depicted above or a pharmaceutically acceptable salt thereof.

The binding affinity (K_(i)) of the compounds according to the present invention for the α3 subunit of the human GABA_(A) receptor is conveniently as measured in the assay described hereinbelow. The α3 subunit binding affinity (K_(i)) of the compounds of the invention is less than 1 nM.

The compounds according to the present invention elicit a selective potentiation of the GABA EC₂₀ response in stably transfected recombinant cell lines expressing the α3 subunit of the human GABA_(A) receptor relative to the potentiation of the GABA EC₂₀ response elicited in stably transfected recombinant cell lines expressing the α1 subunit of the human GABA_(A) receptor.

The potentiation of the GABA EC₂₀ response in stably transfected cell lines expressing the α3 and α1 subunits of the human GABA_(A) receptor can conveniently be measured by procedures analogous to the protocol described in Wafford et al., Mol. Pharmacol., 1996, 50, 670-678. The procedure will suitably be carried out utilising cultures of stably transfected eukaryotic cells, typically of stably transfected mouse Ltk⁻ fibroblast cells.

The compounds according to the present invention exhibit anxiolytic activity, as demonstrated by a positive response in the elevated plus maze and conditioned suppression of drinking tests (cf. Dawson et al., Psychopharmacology, 1995, 121, 109-117). Moreover, the compounds of the invention are substantially non-sedating, as confirmed by an appropriate result obtained from the response sensitivity (chain-pulling) test (cf. Bayley et al., J. Psychopharmacol., 1996, 10, 206-213).

The compounds according to the present invention may also exhibit anticonvulsant activity. This can be demonstrated by the ability to block pentylenetetrazole-induced seizures in rats and mice, following a protocol analogous to that described by Bristow et al. in J. Pharmacol. Exp. Ther., 1996, 279, 492-501.

Since they elicit behavioural effects, the compounds of the invention plainly are brain-penetrant; in other words, these compounds are capable of crossing the so-called “blood-brain barrier”. Advantageously, the compounds of the invention are capable of exerting their beneficial therapeutic action following administration by the oral route.

The invention also provides pharmaceutical compositions comprising one or more compounds of this invention in association with a pharmaceutically acceptable carrier. Preferably these compositions are ill unit dosage forms such as tablets, pills, capsules, powders, granules, sterile parenteral solutions or suspensions, metered aerosol or liquid sprays, drops, ampoules, auto-injector devices or suppositories; for oral. parenteral, intranasal, sublingual or rectal administration, or for administration by inhalation or insufflation. For preparing solid compositions such as tablets, the principal active ingredient is mixed with a pharmaceutical carrier, e.g. conventional tableting, ingredients such as corn starch, lactose, sucrose, sorbitol, talc, stearic acid, magnesium stearate, dicalcium phosphate or gums, and other pharmaceutical diluents, e.g. water, to form a solid preformulation composition containing a homogeneous mixture of a compound of the present invention. or a pharmaceutically acceptable salt thereof. When referring to these preformulation compositions as homogeneous, it is meant that the active ingredient is dispersed evenly throughout the composition so that the composition may be readily subdivided into equally effective unit dosage forms such as tablets, pills and capsules. This solid preformulation composition is then subdivided into unit dosage forms of the type described above containing from 0.1 to about 500 mg of the active ingredient of the present invention. Typical unit dosage forms contain from 1 to 100 mg, for example 1, 2, 5, 10, 25, 50 or 100 mg, of the active ingredient. The tablets or pills of the novel composition can be coated or otherwise compounded to provide a dosage form affording the advantage of prolonged action. For example, the tablet or pill can comprise an inner dosage and an outer dosage component, the latter being in the form of an envelope over the former. The two components can be separated by an enteric layer which serves to resist disintegration in the stomach and permits the inner component to pass intact into the duodenum or to be delayed in release. A variety of materials can be used for such enteric layers or coatings, such materials including a number of polymeric acids and mixtures of polymeric acids with such materials as shellac, cetyl alcohol and cellulose acetate.

The liquid forms in which the novel compositions of the present invention may be incorporated for administration orally or by injection include aqueous solutions, suitably flavoured syrups, aqueous or oil suspensions, and flavoured emulsions with edible oils such as cottonseed oil, sesame oil, coconut oil or peanut oil, as well as elixirs and similar pharmaceutical vehicles. Suitable dispersing or suspending agents for aqueous suspensions include synthetic and natural gums such as tragacanth, acacia, alginate, dextran, sodium carboxymethylcellulose, methylcellulose, polyvinyl-pyrrolidone or gelatin.

In the treatment of anxiety, a suitable dosage level is about 0.01 to 250 mg/kg per day, preferably about 0.05 to 100 mg/kg per day, and especially about 0.05 to 5 mg/kg per day. The compounds may be administered on a regimen of 1 to 4 times per day.

The compound of formula I as depicted above may be prepared by a process which comprises reacting a compound of formula III with the compound of formula IV:

wherein L¹ represents a suitable leaving group.

The leaving group L¹ is typically a halogen atom, especially chloro.

The reaction between compounds III and IV is conveniently effected by stirring the reactants in a suitable solvent. in the presence of a base. Typically, the solvent is NN-dimethylformamide, and the base is a strong base such a sodium hydride. Preferably, the solvent is dimethyl sulfoxide, and the base is cesium carbonate. More preferably, the solvent is 1-methyl-2-pyrrolidinone, and the base is sodium hydroxide, in which case the reaction is advantageously performed at a temperature in the region of 0° C.

The intermediates of formula III above may be prepared bv reacting a compound of formula V with a substantially equimolar amount of the hydrazine derivative of formula VI:

wherein L¹ is as defined above, and L² represents a suitable leaving group; followed, if necessary, by separation of the resulting mixture of isomers by conventional means.

The leaving group L² is typically a halogen atom, especially chloro. In the intermediates of formula V, the leaving groups L¹ and L² may be the same or different, but are suitably the same, preferably both chloro.

The reaction between compounds V and VI is conveniently effected by heating the reactants in the presence of a proton source such as triethylamine hydrochloride, typically at reflux in an inert solvent such as xylene or 1,4-dioxane.

Alternatively, the intermediates of formula III above may be prepared by reacting a hydrazine derivative of formula VII with the aldehyde derivative of formula VIII:

wherein L¹ is as defined above; followed by cyclization of the intermediate Schiffs base thereby obtained.

The reaction between compounds VII and VIII is conveniently effected under acidic conditions, for example in the presence of a mineral acid such as hydrochloric acid. Cyclization of the resulting Schiffs base intermediate may then conveniently be carried out by treatment with iron(III) chloride in a suitable solvent, e.g. an alcoholic solvent such as ethanol, at an elevated temperature, typically at a temperature in the region of 60-70° C.

The intermediates of formula VII above may be prepared by reacting the appropriate compound of formula V as defined above with hydrazine hydrate, typically in isobutyl alcohol at an elevated temperature, e.g. a temperature in the region of 90° C., or in 1,4-dioxane at the reflux temperature of the solvent; followed, if necessary, by separation of the resulting mixture of isomers by conventional means.

In an alternative approach, the intermediates of formula III above may be prepared by reacting the hydrazine derivative of formula VII as defined above with a compound of formula IX:

wherein Q represents a reactive carboxylate moiety: followed by cyclization of the hydrazide derivative of formula X thereby obtained:

wherein L¹ is as defined above.

Suitable values for the reactive carboxylate moiety Q include esters, for example C₁₋₄ alkyl esters; acid anhydrides, for example mixed anhydrides with C₁₋₄ alkanoic acids; acid halides, for example acid chlorides; and acylimidazoles. Suitably, Q represents an acid chloride moiety.

The reaction between compounds VII and IX is conveniently effected under basic conditions, e.g. in the presence of triethylamine, suitably in an inert solvent such as diethyl ether, and typically at a temperature in the region of 0° C. Cyclization of the resulting compound of formula X may then conveniently be carried out by treatment with 1,2-dibromo-1,1,2,2-tetrachloroethane and triphenylphosphine, in the presence of a base such as triethylamine, suitably in an inert solvent such as acetonitrile, and typically at a temperature in the region of 0° C.

In a preferred method, the reaction between compounds VII and IX may be effected by mixing the reactants in a solvent such as 1-methyl-2-pyrrolidinone at a temperature in the region of 0° C.; cyclization of the compound of formula X thereby obtained may then be accomplished in situ by heating the reaction mixture at a temperature in the region of 130° C.

The reaction between compound V and hydrazine hydrate or compound VI will, as indicated above, possibly give rise to a mixture of isomeric products depending upon whether the hvdrazine nitrogen atom displaces the leaving group L¹ or L². Thus, in addition to the required product of formula III or VII, the alternative isomer will possibly be obtained to some extent. For this reason it might be necessary to separate the resulting mixture of isomers by conventional methods such as chromatography.

In another procedure, the compound of formula I as depicted above may be prepared by a process which comprises reacting the compound of formula XI (or its 1,2,4-triazolo[4,3-b]pyridazin-6-one tautomer) with a compound of formula XII:

wherein L³ represents a suitable leaving group.

The leaving group L³ is suitably a halogen atom, typically chloro or bromo.

The reaction between compounds XI and XII is conveniently effected by stirring the reactants in a suitable solvent, typically N,N-dimethylformamide, in the presence of a strong base such as sodium hydride.

The intermediate of formula XI above may conveniently be prepared by reacting a compound of formula III as defined above with an alkali metal hydroxide, e.g. sodium hydroxide. The reaction is conveniently effected in an inert solvent such as aqueous 1,4-dioxane, ideally at the reflux temperature of the solvent.

In a further procedure, the compound of formula I as depicted above may be prepared by a process which comprises reacting trimethylacetic acid with the compound of formula XIII:

in the presence of silver nitrate and ammonium persulphate.

The reaction is conveniently carried out in a suitable solvent, for example in water or aqueous acetonitrile, optionally under acidic conditions, e.g. using trifluoroacetic acid or sulphuric acid, typically at an elevated temperature.

The intermediate of formula XIII corresponds to the compound of formula I as depicted above wherein the tert-butyl substituent at the 7-position is absent, and the intermediate XIII may therefore be prepared by methods analogous to those described above for preparing the compound of formula I.

In a still further procedure, the compound of formula I as depicted above may be prepared by a process which comprises reacting a compound of formula XIV with a compound of formula XV:

wherein M represents —B(OH)₂ or —Sn(Alk)₃ in which Alk represents a C₁₋₆ alkyl group, typically n-butyl, and L⁴ represents a suitable leaving group; in the presence of a transition metal catalyst.

The leaving group L⁴ is suitably a halogen atom, e.g. bromo.

A suitable transition metal catalyst of use in the reaction between compounds XIV and XV comprises dichlorobis(triphenylphosphine)-palladium(II) or tetrakis(triphenylphosphine)palladium(0).

The reaction between compounds XIV and XV is conveniently effected in an inert solvent such as N,N-dimethylformamide, typically at an elevated temperature.

The intermediates of formula XIV may be prepared by reacting the compound of formula IV as defined above with a compound of formula XVI:

wherein L¹ and L⁴ are as defined above; under conditions analogous to those described above for the reaction between compounds III and IV.

The intermediate of formula IV above may be prepared by the procedures described in EP-A-0421210, or by methods analogous thereto. Suitable methods are described in the accompanying Examples.

The intermediates of formula V above may be prepared by reacting trimethylacetic acid with a compound of formula XVII:

wherein L¹ and L² are as defined above; in the presence of silver nitrate and ammonium persulphate; under conditions analogous to those described above for the reaction between trimethylacetic acid and compound XIII. Where L¹ and L² are both chloro in compound XVII, the reaction is advantageously effected in the presence of trifluoroacetic acid.

Where they are not commercially available, the starting materials of formula VI, VIII, IX, XII, XV, XVI and XIII may be prepared by methods analogous to those described in the accompanying Examples, or by standard methods well known from the art.

During any of the above synthetic sequences it may be necessary and/or desirable to protect sensitive or reactive groups on any of the molecules concerned. This may be achieved by means of conventional protecting groups, such as those described in Protective Groups in Organic Chemistry, ed. J. F. W. McOmie, Plenum Press, 1973; and T. W. Greene & P. G. M. Wuts, Protective Groups in Organic Synthesis, John Wiley & Sons, 1991. The protecting groups may be removed at a convenient subsequent stage using methods known from the art.

Four anhydrous polymorphs, two solvates and a dihydrate of 7-(1,1-dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine have been synthesised and characterised. All the polymorphs and solvates revert to the most thermodynamically stable form, polvmorph A (for formation and characterisation see Example 3), after stirring as a suspension in water. A dihydrate of polymorph A is stable, but only at elevated humidities.

The following Examples illustrate the preparation of compounds according to the invention.

The compounds in accordance with this invention potently inhibit the binding of [³H]-flumazenil to the benzodiazepine binding site of human GABA_(A) receptors containing the α2 or α3 subunit stably expressed in Ltk⁻ cells.

Reagents

Phosphate buffered saline (PBS).

Assay buffer: 10 mM KH₂PO₄, 100 mM KCl, pH 7.4 at room temperature.

[³H]-Flumazenil (18 nM for α1β3γ2 cells; 18 nM for α2β3γ2 cells; 10 nM for α3β3γ2 cells) in assay buffer.

Flunitrazepam 100 μM in assay buffer.

Cells resuspended in assay buffer (1 tray to 10 ml).

Harvesting Cells

Supernatant is removed from cells. PBS (approximately 20 ml) is added. The cells are scraped and placed in a 50 ml centrifuge tube. The procedure is repeated with a further 10 ml of PBS to ensure that most of the cells are removed. The cells are pelleted by centrifuging for 20 min at 3000 rpm in a benchtop centrifuge, and then frozen if desired. The pellets are resuspended in 10 ml of buffer per tray (25 cm×25 cm) of cells.

Assay

Can be carried out in deep 96-well plates or in tubes. Each tube contains:

300 μl of assay buffer.

50 μl of [³H]-flumazenil (final concentration for α1β3γ2: 1.8 nM; for α2β3γ2: 1.8 nM; for α3β3γ2: 1.0 nM).

50 μl of buffer or solvent carrier (e.g. 10% DMSO) if compounds are dissolved in 10% DMSO (total); test compound or flunitrazepam (to determine non-specific binding), 10 μM final concentration.

100 μl of cells.

Assays are incubated for 1 hour at 40° C., then filtered using either a Tomtec or Brandel cell harvester onto GF/B filters followed by 3×3 ml washes with ice cold assay buffer. Filters are dried and counted by liquid scintillation counting. Expected values for total binding are 3000-4000 dpm for total counts and less than 200 dpm for non-specific binding if using liquid scintillation counting, or 1500-2000 dpm for total counts and less than 200 dpm for non-specific binding if counting with meltilex solid scintillant. Binding parameters are determined by non-linear least squares regression analysis, from which the inhibition constant K_(i) can be calculated for each test compound.

The compound of the accompanying Examples was tested in the above assay, and was found to possess a K_(i) value for displacement of [3H]-flumazenil from the α2 and/or α3 subunit of the human GABA_(A) receptor of less than 1 nM.

EXAMPLE 1 7-(1,1-Dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine

a) 3,6-Dichloro-4-(1,1-dimethylethyl)pyridazine

Concentrated sulphuric acid (53.6 ml, 1.0 mol) was added carefully to a stirred suspension of 3,6-dichloropyridazine (50.0 g, 0.34 mol) in water (1.25 l). This mixture was then heated to 70° C. (internal temperature) before the addition of trimethylacetic acid (47.5 ml, 0.41 mol). A solution of silver nitrate (11.4 g, 0.07 mol) in water (20 ml) was then added over approximately one minute. This caused the reaction mixture to become milky in appearance. A solution of ammonium persulphate (230 g, 1.0 mol) in water (0.63 l) was then added over 20-30 minutes. The internal temperature rose to approximately 85° C. During the addition the product formed as a sticky precipitate. Upon complete addition the reaction was stirred for an additional 10 minutes, then allowed to cool to room temperature. The mixture was then poured onto ice and basified with concentrated aqueous ammonia, with the addition of more ice as required to keep the temperature below 10° C. The aqueous was extracted with dichloromethane (3×300 ml). The combined extracts were dried (MgSO₄), filtered and evaporated to give 55.8 g of crude product as an oil. This was purified by silica gel chromatography using 0-15% ethyl acetate in hexane as eluent to give 37.31 g (53%) of the desired compound. Data for the title compound: ¹H NMR (360 MHz, d₆-DMSO) δ1.50 (9H, s), 7.48 (1H, s); MS (ES⁺) m/e 205 [MH]⁺, 207 [MH]⁺.

b) 6-Chloro-7-(1,1-dimethylethyl)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine

A mixture of 3,6-dichloro-4-(1,1-dimethylethyl)pyridazine (20 g, 0.097 mol), 2-fluorobenzhydrazide (22.6 g, 0.145 mol) and triethylamine hydrochloride (20 g, 0.0145 mol) in dioxan (1.2 l) was stirred and heated at reflux under a stream of nitrogen for 4 days. Upon cooling the volatiles were removed in vacuo and the residue was triturated with dichloromethane (200 ml), filtered and concentrated under vacuum. The residue was purified by chromatography on silica gel eluting with 0%→25% ethyl acetate/dichloromethane to give the title compound (12.95 g. 44%) as a white solid. Data for the title compound: ¹H NMR (360 MHz, CDCl₃) δ1.57 (9H, s), 7.26-7.35 (2H, m), 7.53-7.60 (1H, m), 7.89-7.93 (1H, m), 8.17 (1H, s); MS (ES⁺) m/e 305 [MH]⁺, 307 [MH]⁺.

c) (2-Ethyl-2H-1,2,4-triazol-3-yl)methanol

To a solution of 1,2,4-triazole (10 g, 0.145 mol) in DMF (150 ml) at room temperature was added sodium hydride (6.4 g of a 60% disp. in oil, 0.16 mol) in portions over 15 min. When the addition was complete, the reaction mixture was allowed to cool to room temperature, then cooled in an ice-bath and iodoethane (14 ml, 0.174 mol) added dropwise over 10 min. The reaction mixture was allowed to warm to room temperaure and after stirring for 3 h the solvents were removed under high vacuum to leave a residue which was partitioned between water (300 ml) and ethyl acetate (3×300 ml). The combined organic layers were washed with saturated brine and dried (MgSO₄), filtered and concentrated under vacuum to leave an oily residue which was purified by distillation (120° C. @ ˜20 mmHg) to give 1-ethyl-1,2,4-triazole contaminated with ˜15% DMF (2.4 g). The crude product (2.4 g, 0.025 mol) was dissolved in dry THF (35 ml), cooled to −40° C. and n-butyllithium (16.2 ml of a 1.6 M solution in hexane, 0.026 mol) was added slowly over 20 min keeping the temperature constant. DMF (2.03 ml, 0.026 mol) was then added and after 15 min the reaction mixture was allowed to warm slowly to room temperature over 2 h. To the reaction mixture was added methanol (20 ml) followed by sodium borohydride (1 g, 0.026 mol) and the solution was allowed to stir for 14 h. The solvents were removed under vacuum and the residue was partitioned between brine (50 ml) and dichloromethane (6×50 ml). The combined organic layers were dried (MgSO₄), filtered and concentrated under vacuum to leave a residue which was purified by silica gel chromatography using 0-5% methanol in dichlorometane as eluent to give the title compound as an off-white solid (0.5 g, 3%). Data for the title compound: ¹H NMR (250 MHz, CDCl₃) δ1.48 (3H, t, J=7.3 Hz), 4.25 (2H, q, J=7.3 Hz), 4.75 (2H, s), 5.14 (1H, br s), 7.78 (1H, s).

d) 7-(1,1-Dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine

To a solution of (2-ethyl-2H-1,2,4-triazol-3-yl)methanol (0.094 g, 0.74 mmol) and 6-chloro-7-(1,1-dimethylethyl)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine (0.15 g. 0.49 mmol) in DMF (10 ml) was added sodium hydride (0.024 g of a 60% dispersion in oil, 1.1 mol eq.) and the reaction mixture was stirred at room temperature for 30 minutes. After this time, the reaction mixture was diluted with water (80 ml) and the solid that precipitated was collected by filtration and washed several times with water in the sinter funnel. The solid was recrystallised from ethyl acetate/hexane to give pure title compound (0.085 g, 44%). Data for the title compound: ¹H NMR (250 MHz, CDCl₃) δ1.40-1.47 (12H, m), 4.14 (2H, t, J=7.3 Hz), 5.26 (2H, s), 7.26-7.38 (2H, m), 7.53-7.58 (1H, m), 7.86-7.90 (1H, m), 7.93 (1H, s), 7.99 (1H, s); MS (ES⁺) m/e 396 [MH]⁺. Anal. Found C, 61.02; H, 5.45; N, 24.75%. C₂₀H₂₂FN₇O requires C, 60.75; H, 5.61; N, 24.79%.

EXAMPLE 2 7-(1,1-Dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pvridazine: Alternative Synthetic Route

a) 3,6-Dichloro-4-(1,1-dimethylethyl)pyridazine

A mixture of dichloropyridazine (100 g, 0.67 mol), trimethylacetic acid (96 g, 0.94 mol) and water (800 ml) in a 10 l flask with overhead stirring was warmed to 55° C. resulting in a two-phase solution. A solution of AgNO₃ (11.4 g, 0.134 mol) in water (125 ml) was added in one portion resulting in an opaque solution. Trifluoroacetic acid (10.3 ml, 0.134 mol) was added in one portion. Ammonium persulfate (245 g, 1.07 mol) was dissolved in water (500 ml) and added dropwise to the suspension over 45-60 min resulting in an exotherm (typically temperature increases to between 75-80° C. and can be controlled by rate of persulfate addition). The temperature was maintained at 75° C. for a further 1 h and then cooled to room temperature. The reaction mixture was extracted with isobutyl alcohol (1 l) and the aqueous layer discarded. The organic layer was washed with water (250 ml) and the aqueous cut discarded. HPLC assay yield is 134 g (97%). The isobutyl alcohol solution was used as is in the next step.

b) 6-Chloro-5-(1,1-dimethylethyl)pyridazin-3-ylhydrazine

Hydrazine hydrate (95 ml, 1.95 mol) was added to the isobutyl alcohol solution from step a in a 3 l flask and heated at 90° C. for 20 h. The reaction mixture was cooled to room temperature and the lower aqueous layer discarded. The reaction mixture was washed with water (450 ml) and the aqueous cut discarded. The reaction mixture was distilled under reduced pressure until the product started to crystallise and then 1-methyl-2-pyrrolidinone (NMP) (550 ml) was added. Distillation was continued to remove the last of the isobutyl alcohol. The solution was used as is in the next step.

c) 6-Chloro-7-(1,1-dimethylethyl)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]-pyridazine

2-Fluorobenzoyl chloride (103 g, 0.65 mol) was added dropwise to the cooled (0° C.) NMP solution from step b maintaining the internal temperature<5° C. After the addition the reaction mixture was heated at 130° C. for 2 h. The reaction mixture was cooled to room temperature which resulted in the crystallisation of the product. Water (1.3 l) was added dropwise over 30 min. The slurry was cooled to 10° C. and the solid isolated by filtration, then dried under reduced pressure to afford product (145 g, 71% yield from 3,6-dichloropyridazine).

d) 1-Ethyl-1,2,4-triazole

1,2,4-Triazole (100.0 g, 1.45 mol) in anhydrous THF (950 ml) was cooled to 0° C. and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (220 g, 1.45 mol) was added in one portion. The reaction mixture was stirred for 30 min until complete dissolution was observed. Whilst maintaining the ice/water cooling bath, iodoethane (317 g, 2.03 mol) was added dropwise over a 15 min period resulting in an internal temperature rise to 30° C. The reaction was stirred at room temperature for 16 h, after which the DBU hydroiodide was removed by filtration. The filtrate solution was used as is in the next step.

e) (2-Ethyl-2H-1,2,4-triazol-3-yl)methanol

The stirred solution from the preceding step was cooled to −75° C. internal temperature in a solid CO₂/acetone slurry bath. Hexyllithium (458 ml of 33% solution in hexanes) was added dropwise over 25 min keeping the internal temperature below −55° C. The reaction mixture was aged for 30 min (back to −75° C.) and then neat DMF (108 ml, 1.39 mol) was added dropwise over 10 min maintaining internal temperature below −60° C. The reaction mixture was aged at −70° C. for 90 min before the cooling bath was removed and the reaction mixture allowed to warm to 0° C. over 30 min. Industrial methylated spirits (340 ml) was added over 10 min. Sodium borohydride (26.3 g, 0.695 mol) was then added portionwise maintaining internal temperature below 6° C. After the addition the reaction mixture was allowed to warm to room temperature and stirred for 1 h at that temperature. The reaction was quenched by the careful addition of 2M H₂SO₄ (200 ml) and then stirred at room temperature for 20 h. The reaction mixture was concentrated to 675 ml and sodium sulfate (135 g) was added in one portion. The reaction mixture was warmed to 35° C. and stirred for 15 min. The solution was extracted with warm (45° C.) isobutyl alcohol (2×675 ml). The combined organic fractions were concentrated under reduced pressure 450 ml at which point the product crystallised. Heptane (1.125 l) was added and the slurry concentrated under reduced pressure to remove most of the isobutyl alcohol. Heptane was added to give a final slurry volume of 680 ml. After cooling to 0° C., filtration gave the title compound (137 g, 74% from 1,2,4-triazole).

f) 7-(1,1-Dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazol[4,3-b]pyridazine

Method A

6-Chloro-7-(1,1-dimethylethyl)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine (255 g, 0.819 mol), (2-ethyl-2H-1,2,4-triazol-3-yl)methanol (125 g, 0.983 mol) and cesium carbonate (640 g, 1.966 mol) were charged to a 10 l flask equipped with an overhead stirrer. Dimethyl sulfoxide (2.5 l) was added in one portion and the reaction mixture stirred at room temperature for 20 h. The product had crystallised. The reaction mixture was maintained at <25° C. whilst water (5 l) was added dropwise over 45 min to the stirred suspension. After cooling to 10° C., the product was isolated by filtration and the cake washed with water (1.75 l). Drying at 50° C. in vacuo gave the title compound (317 g, 98%) as a white solid.

Method B

6-Chloro-7-(1,1-dimethylethyl)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine (10 g, 32.12 mmol) and (2-ethyl-2H-1,2,4-triazol-3-yl)methanol (5.01 g, 38.55 mmol) were charged to a 500 ml flask equipped with an overhead stirrer. NMP (100 ml) was added in one portion and the reaction mixture stirred until complete dissolution was achieved. The reaction mixture was cooled to 0° C. and 48% w/w sodium hydroxide solution (4.02 g, 46 mmol) was added in one portion. After stirring for 1 h at 0° C., the product had crystallised. Water (100 ml) was added dropwise over 15 min and the slurry aged for 30 min. The product was isolated by filtration and the cake washed with water (100 ml). Drying at 50° C. in vacuo gave the title compound (12.50 g. 98%) as a white solid.

EXAMPLE 3 Formation and Characterisation of Polymorphs and Solvates of 7-(1,1-dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine

7-(1,1-Dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine (hereinafter referred to as Compound I) was recrystallised from a selection of organic solvents and the resultant solids were dried in vacuo overnight at 60° C. unless otherwise stated. Each batch was then characterised by optical microscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and X-ray powder diffraction (XRPD). Four distinct anhydrous polymorphs, a hydrate and two solvates have been characterised, as summarised in Table 1.

TABLE 1 Polymorphs of Compound I Recrystallisation Crystal Polymorph Solvent Habit M.P. ° C. A Methanol Elongated ca. 186 Ethyl acetate rectangular or Acetonitrile irregular Acetone Acetic acid/H₂O B Isobutanol Elongated ca. 181 rectangular or irregular C Dimethylformamide Acicular ca. 170 Tetrahydrofuran D Isopropanol Acicular ca. 170 Isobutanol Methanol Solvate Methanol (air-dried) Irregular ca. 186 Ethanol Solvate Ethanol Irregular ca. 186

Polymorph A

Polymorph A consists of irregular or elongated rectangular birefringent crystals. It shows one major endotherm by DSC at ca. 186° C. due to melting. In the case of some of the samples prepared this melt displays several events happening within it, typified by shoulders on the melting endotherm. Polymorph A is anhydrous and displays no losses by TGA. It has a unique XRPD diffractogram, typified by two peaks at 2θ≈7.3°.

Polymorph B

Polymorph B consists of irregular or elongated rectangular birefringent crystals. Polymorph B displays one major endotherm by DSC at ca. 181° C. due to melting. It is anhydrous and displays no weight loss by TGA. It has a unique XRPD diffractogram.

Polymorph C

Polymorph C consists of acicular, birefringent crystals. The DSC thermograms of polymorph C show an endotherm at ca. 170° C., an exotherm at ca. 173° C., a small endotherm at ca. 181° C., and an endotherm due to melting at ca. 186° C. No losses are observed by thermogravimetric analysis.

Polymorph D

Polymorph D consists of acicular birefringent crystals. The XRPD diffractogram of polymorph D is similar in pattern to that of polymorph C. However, significant differences are observed, notably additional peaks at 2θ=9.861, 15.113, 18.015 and 22.224. The DSC trace of polymorph D displays a very broad exotherm at ca. 108° C., followed by an endotherm and exotherm at 170° C. and 173° C., as for polymorph C. The major melting is observed at ca. 181° C., and several events are seen to occur within the melting endotherm, with minor melting at 186° C. No losses are observed by TGA.

Methanol Solvate

The DSC thermogram of the methanol solvate shows one endotherm at ca. 157° C. due to loss of methanol, and one endotherm at 186° C. due to melting. A gradual loss is observed by TGA up to ca. 150° C., at which point a step loss is observed, coincident with the endotherm observed by DSC. This step loss is in some instances composed of more than one event, and the amount varies among recrystallised samples and does not appear to correspond to a stoichiometric solvate. However, methanol vapour sorption studies on polymorph A show the presence of a distinct hemisolvate. The XRPD diffractogram of the solvate is unique.

Ethanol Solvate

The ethanol solvate is typified by its DSC thermogram which shows one endotherm at ca. 111° C. due to loss of ethanol, and an endotherm at ca. 186° C. due to melting. TGA shows the loss to be variable in different recrystallised samples, between ca. 4 and 6.5%. and does not correspond to a stoichiometric solvate. The XRPD diffractogram is unique.

Hydrate of Polymorph A

FIG. 1 shows the adsorption/desorption isotherm of Compound I polymorph A at 25° C. These moisture vapour sorption studies show formation of a dihydrate above 80% relative humidity (RH) at 25° C. Hysteresis indicative of hydrate formation is seen to occur, with desorption occurring below ca. 60% RH.

FIG. 2 shows side-by-side XRPD diffractograms of anhydrous Compound I polymorph A and of the dihydrate of polymorph A. The XRPD diffractogram of the dihydrate, obtained on a wetted sample of polymorph A, demonstrates the distinct differences observed between the two forms. The major changes in diffraction can be seen to be loss of.the peak at 2θ=11.2°, and the appearance of two major peaks at 20=11.9° and 12.3°.

Conversion of Compound I Polymorphs to Polymorph A

On slurrying in water all of the polymorphs and solvates decribed above convert to polymorph A over a period of 1-4 days, indicating that this is the most stable form at room temperature. This conversion is slow due to the low aqueous solubility of Compound I in relation to the large excess of solid compound slurried.

X-Ray Powder Diffraction Data

FIG. 3 shows side-by-side XRPD diffractograms of anhydrous Compound I polymorphs A, B, C and D, of the methanol and ethanol solvates. Numerical data associated therewith are presented below.

Compound I Polymorph A: X-Ray Powder Diffraction Data Angle d value Intensity Intensity Angle d value Intensity Intensity 2θ Å Count % 2θ Å Count % 7.243 12.19523 2214 88.5 22.885 3.88289 195 7.8 7.559 11.68660 613 24.5 23.621 3.76351 307 12.3 8.601 10.27227 50 2 23.67 3.75587 256 10.2 9.694 9.11647 211 8.4 23.767 3.74068 253 10.1 10.703 8.25928 270 10.8 24.12 3.68684 102 4.1 11.027 8.01719 213 8.5 24.518 3.62778 281 11.2 11.239 7.86640 1464 58.5 25.223 3.52803 162 6.5 12.943 6.83420 2501 100 25.275 3.52082 185 7.4 13.441 6.58218 41 1.6 25.478 3.49324 808 32.3 14.497 6.10492 740 29.6 26.088 3.41292 2178 87.1 15.178 5.83251 1211 48.4 26.686 3.33787 86 3.4 15.376 5.75802 187 7.5 27.093 3.28857 138 5.5 15.42 5.74169 144 5.8 27.74 3.21330 186 7.4 15.854 5.58536 391 15.6 28.056 3.17789 87 3.5 16.081 5.50706 236 9.4 28.607 3.11788 248 9.9 16.134 5.48931 250 10 28.703 3.10770 155 6.2 16.419 5.39437 135 5.4 29.135 3.06255 131 5.2 16.76 5.28552 36 1.4 29.267 3.04911 284 11.4 17.165 5.16167 304 12.1 29.353 3.04034 193 7.7 17.199 5.15149 235 9.4 29.787 2.99703 71 2.8 17.24 5.13933 329 13.2 30.357 2.94205 106 4.2 17.355 5.10550 281 11.2 30.954 2.88666 38 1.5 18.556 4.77774 213 8.5 31.436 2.84343 52 2.1 18.829 4.70917 85 3.4 32.618 2.74309 163 6.5 19.27 4.60224 372 14.9 32.773 2.73047 161 6.5 19.499 4.54892 233 9.3 33.254 2.69202 64 2.6 20.203 4.39192 118 4.7 33.897 2.64245 66.4 2.7 20.635 4.30098 238 9.5 34.14 2.62417 162 6.5 21.584 4.11385 250 10 36.522 2.45827 53 2.1 21.689 4.09416 704 28.2 38.532 2.33459 40 1.6 21.867 4.06119 1621 64.8 38.859 2.31564 78 3.1 22.28 3.98692 154 6.2 39.594 2.27434 47 1.9

Compound I Polymorph B: X-Ray Powder Diffraction Data Angle d value Intensity Intensity Angle d value Intensity Intensity 2θ Å Count % 2θ Å Count % 7.379 11.97023 651 49.2 20.179 4.39705 119 9 10.096 8.75424 481 36.4 20.9 4.24699 54 4.1 10.821 8.16974 1323 100 21.718 4.08884 1300 98.3 12.482 7.08575 61 4.6 22.285 3.98602 446 33.7 12.739 6.94328 149 11.3 22.96 3.87033 87 6.6 13.036 6.78588 194 14.7 23.304 3.81394 111 8.4 13.475 6.56586 123 9.3 24.018 3.70224 73 5.5 13.828 6.39890 69 5.2 24.459 3.63648 75 5.7 14.815 5.97480 322 24.3 25.081 3.54765 141 10.7 15.078 5.87109 307 23.2 25.837 3.44555 47 3.6 15.584 5.68147 86 6.5 26.204 3.39810 193 14.6 16.082 5.50675 391 29.6 27.078 3.29031 176 13.3 16.979 5.21789 147 11.1 28.449 3.13490 91 6.9 17.02 5.20548 163 12.3 28.889 3.08812 51 3.9 17.287 5.12570 257 19.4 29.84 2.99177 166 12.6 17.616 5.03052 72 5.4 29.922 2.98380 121 9.1 18.125 4.89033 83.1 6.3 30.227 2.95440 58 4.4 18.161 4.88081 129 9.7 31.94 2.79975 70.5 5.3 18.192 4.87269 143 10.8 32.837 2.72528 117 8.8 19.022 4.66174 69 5.2 33.295 2.68885 40 3 19.542 4.53891 119 9 34.407 2.60440 52 3.9 19.68 4.50748 70.5 5.3 38.876 2.31472 27 2

Compound I Polymorph C: X-Ray Powder Diffraction Data Angle d value Intensity Intensity Angle d value Intensity Intensity 2θ Å Count % 2θ Å Count % 7.254 12.17653 25 0.6 24.647 3.60908 75 1.8 8.023 11.01169 798 19.3 24.961 3.56448 398 9.6 9.686 9.12421 4130 100 25.341 3.51181 182 4.4 10.489 8.42726 64 1.5 25.549 3.48378 617 14.9 10.813 8.17559 107 2.6 25.981 3.42671 832 20.1 11.277 7.83989 31 0.8 26.794 3.32466 328 7.9 12.193 7.25327 48 1.2 27.782 3.20863 166 4 12.961 6.82506 40 1 27.985 3.18576 158 3.8 14.357 6.16430 146 3.5 28.257 3.15573 241 5.8 14.497 6.10512 213 5.2 28.788 3.09868 257 6.2 15.225 5.81495 35 0.8 29.242 3.05164 322 7.8 16.007 5.53248 917 22.2 29.301 3.04560 322 7.8 16.545 5.35385 681 16.5 29.56 3.01950 111 2.7 16.951 5.22646 61 1.5 30.544 2.92444 158 3.8 17.672 5.01479 402 9.7 30.942 2.88768 70 1.7 18.337 4.83428 536 13 31.839 2.80834 156 3.8 18.578 4.77228 533 12.9 32.457 2.75628 157 3.8 19.396 4.57272 3486 84.4 32.745 2.73271 369 8.9 20.034 4.42851 57 1.4 34.246 2.61626 150 3.6 20.913 4.24430 193 4.7 31.979 2.56310 76 1.8 21.189 4.18959 1917 46.4 35.42 2.53225 74 1.8 21.523 4.12543 476 11.5 35.753 2.50941 92 2.2 21.876 4.05969 329 8 36.71 2.44615 91 2.2 22.654 3.92185 151 3.7 37.036 2.42537 145 3.5 23.207 3.82968 463 11.2 37.549 2.39339 137 3.3 23.238 3.82472 462 11.2 38.915 2.31247 74 1.8 24.316 3.65757 87 2.1 39.41 2.28456 166 4

Compound I Polymorph D: X-Ray Powder Diffraction Data Angle d value Intensity Intensity Angle d value Intensity Intensity 2θ Å Count % 2θ Å Count % 8.024 11.00971 501 33.4 22.224 3.99690 414 27.6 9.664 9.14458 1501 100 22.654 3.92192 112 7.5 9.861 8.96259 502 33.5 23.22 3.82757 322 21.5 10.497 8.42115 38 2.5 24.052 3.69702 89 5.9 10.8 8.18489 229 15.3 24.313 3.65791 295 19.7 11.462 7.71362 40 2.7 24.948 3.56619 272 18.1 12.105 7.30563 216 14.4 25.3 3.51744 120 8 12.951 6.83035 573 38.2 25.543 3.48455 304 20.3 13.312 6.64558 107 7.1 25.977 3.42728 362 24.1 13.817 6.40392 39 2.6 26.036 3.41965 416 27.7 14.478 6.11305 282 18.8 26.789 3.32518 539 35.9 14.823 5.97145 76 5.1 27.479 3.24329 173 11.5 15.113 5.85746 162 10.8 27.754 3.21176 198 13.2 16 5.53485 396 26.4 28.019 3.18199 170 11.3 16.533 5.35742 774 51.6 28.261 3.15525 314 20.9 16.952 5.22613 72 4.8 28.478 3.13170 144 9.6 17.661 5.01784 509 33.9 2.8809 3.09646 205 13.7 18.015 4.92003 479 31.9 29.257 3.05012 176 11.7 18.316 4.83994 545 36.3 29.83 2.99276 178 11.9 18.46 4.80255 512 34.1 30.533 2.92545 157 10.5 18.543 4.78110 498 33.2 30.904 2.89114 96 6.4 19.239 4.60966 196 13.1 31.818 2.81015 131 8.7 19.41 4.56957 775 51.6 32.454 2.75655 119 7.9 20.005 4.43493 158 10.5 32.744 2.73282 182 12.1 20.525 4.32367 82 5.5 33.654 2.66094 94 6.3 20.798 4.26748 386 25.7 34.22 2.61823 95 6.3 20.882 4.25060 416 27.7 34.526 2.59568 80 5.3 21.188 4.18982 1344 89.5 34.934 2.56632 99 6.6 21.509 4.12810 1313 87.5 36.998 2.42775 122 8.1 21.853 4.06378 296 19.7 37.545 2.39366 97 6.5

Compound I Methanol Solvate: X-Ray Powder Diffraction Data Angle d value Intensity Intensity Angle d value Intensity Intensity 2θ Å Count % 2θ Å Count % 7.357 12.00677 9531 90.4 21.153 4.19677 224 2.1 7.608 11.61023 260 2.5 22.081 4.02236 10545  100 9.616 9.19052 260 2.5 22.588 3.93327 526 5 9.755 9.05958 433 4.1 22.825 3.89290 1150  10.9 10.943 8.07881 1145 10.9 23.14 3.84065 362 3.4 11.274 7.84189 406 3.9 23.742 3.74460 291 2.8 12.36 7.15546 3517 33.4 24.295 3.66064 440 4.2 12.942 6.83511 472 4.5 24.818 3.58463 3357  31.8 12.981 6.81447 530 5 25.434 3.49921 933 8.8 14.42 6.13754 440 4.2 25.967 3.42863 2020  19.2 14.69 6.02552 3179 30.1 26 3.42428 2000  19 15.196 5.82583 371 3.5 26.761 3.32860 402 3.8 15.651 5.65758 767 7.3 27.151 3.28169 230 2.2 15.913 5.56476 954 9 28.642 3.11415 273 2.6 16.507 5.36597 379 3.6 28.951 3.08162 298 2.8 16.539 5.35556 426 4 29.56 3.01950 1793  17 16.871 5.25102 1070 10.1 30.365 2.94130 455 4.3 17.226 5.14355 237 2.2 31.319 2.85382 313 3 18.226 4.86354 372 3.5 32.074 2.78834 173 1.6 18.613 4.76331 112 1.1 32.739 2.73323 134 1.3 19.283 4.59933 291 2.8 33.77 2.65210 344 3.3 19.545 4.53827 195 1.8 35.185 2.54858 129 1.2 19.961 4.44452 1703 16.2 37.166 2.41715 145 1.4 20.675 4.29264 161 1.5 37.694 2.38454 218 2.1

Compound I Ethanol Solvate: X-Ray Powder Diffraction Data Angle d value Intensity Intensity Angle d value Intensity Intensity 2θ Å Count % 2θ Å Count % 7.357 12.00679 943 52.5 25.061 3.55045 462 25.7 9.541 9.26191 201 11.2 25.341 3.51184 1795 100 9.809 9.00980 208 11.6 25.712 3.46196 290 16.2 10.951 8.07238 1365 76.1 26.055 3.41713 463 25.8 12.031 7.35019 1500 83.6 26.235 3.39415 401 22.4 12.499 7.07637 115 6.4 26.599 3.34857 178 9.9 14.158 6.25067 245 13.7 27.054 3.29329 159 8.9 14.707 6.01841 236 13.2 28.355 3.14498 311 17.3 15.719 5.63302 428 23.8 28.491 3.13034 190 10.6 15.899 5.56980 657 36.6 28.667 3.11154 150 8.4 16.304 5.43218 869 48.4 29.029 3.07353 142 7.9 16.895 5.24359 1076 60 29.658 3.00975 207 11.5 18.227 4.86334 500 27.9 29.819 2.99388 604 33.7 19.109 4.64073 148 8.2 30.421 2.93596 132 7.4 19.49 4.55085 1045 58.2 30.609 2.91837 133 7.4 20.998 4.22731 211 11.8 31.383 2.84817 131 7.3 21.177 4.19195 168 9.3 32.048 2.79050 154 8.6 21.982 4.04035 1230 68.6 32.116 2.78480 146 8.1 22.124 4.01471 1146 63.9 33.78 2.65134 233 13 22.82 3.89385 904 50.4 35.079 2.55608 119 6.6 23.017 3.86082 475 26.5 35.267 2.54289 123 6.9 23.321 3.81127 295 16.5 36.866 2.43614 89 5 23.459 3.78909 282 15.7 37.84 2.37564 88 4.9 23.961 3.71094 701 39.1 39.182 2.29730 102 5.7 24.159 3.68094 1504 83.8 39.406 2.28477 104 5.8

Compound 1 Hydrate of Polymorph A: X-Ray Powder Diffraction Data Angle d value Intensity Intensity Angle d value Intensity Intensity 2θ Å Count % 2θ Å Count % 7.261 12.16414 447 100 18.657 4.75222 48.4 10.8 9.1 9.70988 25.6 5.7 18.779 4.72155 35.7 8 9.626 9.18058 31 6.9 18.859 4.70171 43.7 9.8 10.796 8.18860 283 63.2 19.276 4.60089 82 18.3 11.858 7.45707 170 38 19.504 4.54766 46 10.3 12.218 7.23806 51.8 11.6 20.22 4.38821 28 0.3 12.261 7.21317 79.4 17.7 20.655 4.29677 133 29.7 12.302 7.18922 78 17.4 21.68 4.09588 163 36.4 12.97 6.82039 173 38.7 21.86 4.06263 223 49.8 14.462 6.11993 132 29.5 22.316 3.98062 88.8 19.8 14.518 6.09637 138 30.8 22.537 3.94207 254 56.7 15.19 5.82798 229 51.2 22.579 3.93478 281 62.7 15.402 5.74824 114 25.6 22.901 3.88013 78 17.4 15.866 5.58141 222 49.6 23.673 3.75541 84 18.8 15.906 5.56750 196 43.9 24.507 3.62946 46 10.3 16.08 5.50763 102 22.7 25.469 3.49445 203 45.4 16.127 5.49137 117 26.1 26.087 3.41311 128 28.6 16.178 5.47423 112 25.1 27.754 3.21175 56 12.5 16.221 5.46004 78 17.4 28.613 3.11725 65 14.5 16.357 5.41482 65.9 14.7 29.291 3.04663 57 12.7 16.418 5.39480 71.3 15.9 29.935 2.98248 43 9.6 17.189 5.15467 146 32.6 32.747 2.73253 63 14.1 17.345 5.10866 105 23.5 34.176 2.62146 32 7.2 18.57 4.77428 71 15.9 38.848 2.31630 31 6.9 

What is claimed is:
 1. A compound which is 7-(1,1-Dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine.
 2. The compound according to claim 1 comprising polymorph A of 7-(1,1-dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine.
 3. A pharmaceutical composition comprising 7-(1,1-dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine and with a pharmaceutically acceptable carrier.
 4. A method for the treatment and/or prevention of anxiety which comprises administering to a patient in need thereof an effective amount of 7-(1,1-dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine.
 5. A process for the preparation of 7-(1,1-dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine, which comprises: (A) reacting a compound of formula III with the compound of formula IV:

wherein L¹ represents a suitable leaving group; or (B) reacting the compound of formula XI (or its 1,2,4-triazolo[4,3-b]pyridazin-6-one tautomer) with a compound of formula XII:

wherein L³ represents a suitable leaving group; or (C) reacting trimethylacetic acid with the compound of formula XIII:

in the presence of silver nitrate and ammonium persulphate; or (D) reacting a compound of formula XIV with a compound of formula XV:

wherein M represents —B(OH)₂ or —Sn(Alk)₃ in which Alk represents a C₁₋₆ alkyl group, and L⁴ represents a suitable leaving group; in the presence of a transition metal catalyst.
 6. A process as claimed in claim 5 wherein reaction (A) is carried out in 1-methyl-2-pyrrolidinone, in the presence of sodium hydroxide, at a temperature in the region of 0° C. 